1) Field of the Invention
The present invention relates to a technology for detecting amount of toner deposit in a color image forming apparatus, a technology for controlling image density using the amount of toner deposit detected, a technology for forming a correcting pattern for correcting misalignment of image forming position, and a technology for correcting the misalignment using the correcting pattern.
2) Description of the Related Art
Recently, a color image forming apparatus of a tandem method, which has such a configuration that image forming units including a plurality of image carriers and development apparatus are arranged parallel to each other at positions facing a transfer belt or an intermediate transfer body, and toner images on the image carriers are sequentially transferred onto recording paper carried on the transfer belt or onto the intermediate transfer body, has been developed (see, for example, Japanese Patent Application Laid-open No. 2001-356541).
In the color image forming apparatus using the tandem method, it is necessary to confirm whether printing is possible in appropriate density without causing a misalignment at the time of starting up the apparatus.
Therefore, in the color image forming apparatus using the tandem method, an image forming apparatus that forms a toner pattern on an image carrier or an intermediate transfer body and uses an optical toner density sensor to control the image density based on a measurement of the amount of toner deposits is currently in use.
Since a size of the image forming apparatus becomes smaller and smaller it is not easy to arrange the toner density sensor flexibly. To reduce the number of sensors to the minimum, a method in which a toner pattern on the intermediate transfer body close to a final image is detected to perform various kinds of control is widely used.
In the color image forming apparatus using the tandem method, since the toner images formed on the image carriers for respective colors can be collectively transferred, the printing speed can be increased. However, compared with the color image forming apparatus using the conventional intermediate transfer body, a color misalignment is likely to occur due to the configuration.
With regard to this kind of technique, a couple of inventions are disclosed in, for example, Japanese Patent Application Laid-open No. 2001-249513 and Japanese Patent Application Laid-open No. 2000-81745, describing a misalignment pattern and a detection method in a tandem-type color image forming apparatus.
With regard to the degree of glossiness of the transfer belt, a technique is disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-194843, for detecting the amount of toner deposits.
However, in an image forming apparatus that uses the optical toner density sensor to control the image density based on the measurement of the amount of toner deposits, there is a problem in performing various kinds of control by detecting a toner pattern on the intermediate transfer body close to the final image. That is, since an intermediate transfer belt comes in contact with photosensitive drums, recording paper, and cleaning blades, the intermediate transfer belt is likely to be damaged, and when the surface of the background to be detected is damaged, the amount of reflected light varies with respect to the light emitting amount of the same optical sensor, causing a detection error.
To prevent the above problem, it can be considered to use an intermediate transfer body having high hardness, but a belt having high hardness has problems in that an image is likely to be scattered, and paper is easily curled.
To increase the detection accuracy of the sensor, the distance from the sensor to a detection target cannot be set too long. Therefore, if a potential of the intermediate transfer belt is high, a potential difference between the sensor and the detection target increases, causing a problem in that the toner adheres on the sensor or the sensor output includes a noise.
Various correction methods have been proposed to correct a color misalignment occurring in the color image forming apparatus using the tandem method. One example is a correction method in which a plurality of respective color line images is formed on the transfer belt, to correct the color misalignment from an absolute position of the line images. When a method of detecting the amount of the color misalignment of each of line images with respect to the reference color line is detected, to correct the out of color registration is adopted, a method of detecting an edge of the line from a reflected light output of light irradiated to the line is used as the specific method. In this method, however, the sampling frequency should be set high (matched with the high speed of the machine), in order to improve the detection accuracy of the edge, and high processing speed is also required, thereby causing a problem in that the cost required for correcting out of color registration increases in proportion to the high speed of the machine.
A method of detecting the edge by a charge coupled device (CCD) sensor having high accuracy and high resolution has been proposed in order to improve the detection accuracy of the edge, but even when such means is used, there are still technical problems such as complication of machinery and a cost increase.
In Japanese Patent Application Laid-open No. 2001-249513, therefore, an invention is disclosed in which after a reference color and a measured color to be corrected having a different pattern pitch are superposed on each other, without detecting the edge of the line, a change in the quantity of light corresponding to a first cycle of the superposed color pattern is detected, and out of color registration between the both colors is detected based on the detection information to correct out of color registration.
On the other hand, an invention is disclosed in Japanese Patent Application Laid-open No. 2000-81745, in which a pattern including a plurality of lines having the same width and line intervals equal to the line width is superposed on the reference color and the color to be corrected, and a density detection value of the density of the superposed pattern is compared with a density D0 in an ideal state when the pattern images are in perfect accord with each other, to correct out of color registration.
In the invention disclosed in Japanese Patent Application Laid-open No. 2001-249513, a deviation in the line reading method (that is, a deviation in the vertical scanning) and a deviation in the horizontal scanning (that is, a skew) can be detected, but it is considered that correction with respect to the deviation in the horizontal scanning is difficult, and a specific method for the correction is not specified therein.
The invention disclosed in Japanese Patent Application Laid-open No. 2000-81745 discloses that the amount of deviation in the horizontal scanning and vertical scanning directions can be detected by creating a single patch as described above. However, the difference between the reference density D0 in the ideal state and the detected value largely changes due to the toner density of the respective colors, the emission current of the light emitting diode (LED), being the sensor, and a detection distance of the sensor (a distance between an object to be measured and the sensor). Further, even when a pattern is created only with the reference color in order to correct the value of the density D0 of the reference pattern (a pattern in which the reference color and the color to be corrected are superposed on each other) by the toner density at that time, since this pattern has a different total thickness of the toner from the reference pattern density D0, and hence these do not become equal, thereby causing a detection error in the correction amount of out of color registration.
An inelastic belt formed by using fluororesin, polycarbonate resin, or polyimide resin has been heretofore used for the intermediate transfer belt corresponding to the background in the misalignment detection. Recently, however, an elastic belt in which elastic members are used for the whole layers of the belt or a part of the belt has been frequently used.
This is because problems described below occur when a color image is transferred by using the inelastic belt (resin belt). That is, a color image is normally formed of colored toners of four colors. Toner layers from a first layer to a fourth layer are formed in one color image. The toner layers are pressured while undergoing primary transfer (transfer from the photosensitive drum to the intermediate transfer belt) and secondary transfer (transfer from the intermediate transfer belt to the sheet), and hence the cohesive power between toners increases. With an increase in the cohesive power between toners, phenomena such as omission in the middle of character and omission of edge in a solid portion of the image are likely to occur. Since the resin belt has high hardness and does not deform according to the toner layer, it easily compresses the toner layer, and the phenomenon of omission in the middle of character is likely to occur.
Recently, there is a high demand to form a full color image on various types of paper, for example, Japanese paper and paper with intentional unevenness. However, with paper having poor smoothness, voids are easily generated between the paper and the toner at the time of transfer, and hence a defect of transferred colorant easily occurs. If the transfer pressure in the secondary transfer unit is increased to increase the adhesion, the cohesive power of the toner layer is increased, thereby causing omission in the middle of character.
On the other hand, the advantages in using the elastic belt are as follows. That is, the elastic belt deforms corresponding to the toner layer and the paper having poor smoothness in the transfer unit. In other words, since the elastic belt deforms, following to the local unevenness, favorable adhesion can be obtained without excessively increasing the transfer pressure with respect to the toner layer, and a transfer image having excellent uniformity with excellent adhesiveness and without omission in the middle of character can be obtained also with respect to the paper having poor smoothness.
However, it is difficult to suppress surface roughness of the elastic belt, due to the characteristic of the material, thereby causing a problem in that S/N in detection by a regular reflection component-detecting type sensor decreases.